1. Field of the Invention
The present invention relates to a catalytic converter, a method for manufacturing the same and a holding and sealing material for catalytic converters.
2. Discussion of the background
Internal combustion engines using gasoline and light oil as fuels have been used for a hundred years or longer as power sources for vehicles, particularly automobiles. However, harmful effects on the health and environment by exhaust gas produced in the internal combustion engine have become a matter of concern. Thus, various kinds of exhaust gas cleaning catalytic converters for removing CO, NOx and PM contained in exhaust gas have been proposed. The exhaust gas cleaning catalytic converter comprises a catalyst carrier, a metal shell covering the outer periphery of the catalyst carrier, and a holding and sealing material disposed in a gap between the catalyst carrier and the metal shell. A honeycomb-shaped cogierite carrier is used for the catalyst carrier, and a catalyst such as platinum is carried on the cogierite carrier.
Recently, studies have been conducted on a fuel cell that is a clean power source using no oil as a power source. In the fuel cell, electricity obtained when hydrogen reacts with oxygen to form water is used as a power source. Oxygen is obtained from air while hydrogen is obtained by reforming methanol or gasoline. Methanol is reformed into hydrogen by a catalyst reaction. In the fuel cell, a catalytic converter for fuel cells comprising the catalyst carrier, the metal shell covering the outer periphery of the catalyst carrier, and the holding and sealing material disposed in a gap between the catalyst carrier and the metal shell. The honeycomb-shaped cogierite carrier is used for the catalyst carrier, and a copper based catalyst is carried on the cogierite carrier.
The method for manufacturing the catalytic converter will be described.
First, a mat material 20 made of ceramic fibers is stamped out to prepare a holding and sealing material 21 having a shape shown in FIG. 20. A holding and sealing material 21 has a convex mating portion 22 formed at a first edge in the longitudinal direction, and a concave mating portion 23 formed at a second edge. The length of the holding and sealing material 21 is slightly shorter than the circumference of a catalyst carrier 24, and its width is almost equal to the thickness of the catalyst carrier 24.
Then, the holding and sealing material 21 is wound around the outer periphery 24a of the catalyst carrier 24. At this time, the both edges of the holding and sealing material 21 are fixed together at several points using an organic tape 25 such as vinyl (see FIG. 21). At this time, the convex mating portion 22 and the concave mating portion 23 engage with each other.
Then, the catalyst carrier 24 with the holding and sealing material 21 wound thereon is disposed in the metal shell bisected in advance, and the catalyst carrier 24 is clamped by the metal shell. In this way, the catalytic converter is completed. This production method is called a canning method. There are assembling methods such as a press fitting method and a clamping method in addition to the canning method.
However, for preparing the holding and sealing material 21 of FIG. 20, stamping using a stamping mold is required for forming convex and concave portions, resulting in poor workability. Also, the stamping of the mat material 20 involves a loss of material, resulting in increased production cost.
In addition, if the size and the sectional form of the catalyst carrier 24 are changed, the stamping form of the holding and sealing material 21 should be changed according to the change, resulting in poor versatility of the holding and sealing material 21. Necessity for a dedicated stamping mold is also responsible for the increased production cost.
In addition, if the density of the holding and sealing material 21 packed in the metal shell is low, the catalyst carrier 24 is easily worn and damaged, and gas is easily leaked from the seal portion. Thus, for solving these problems, the pack density should be increased by making the holding and sealing material 21 thicker as a whole. In this dase, however, the assembling of such as a press fitting and canning cannot easily be performed, thus making it difficult to manufacture the catalytic converter.
The exhaust gas supplied to the catalytic converter has a high temperature of several hundred degrees centigrade or greater, and also tends to generate pulse pressure. Therefore, the end of a gas inlet of the holding and sealing material, which is particularly susceptible to the influence of exhaust gas, is gradually wind-corroded and the holding and sealing material deteriorates. As a result, the carrier holding performance of the holding and sealing material is deteriorated in a relatively short time period, leading to the situation in which the catalyst carrier is easily worn and damaged, or gas is easily leaked from the seal portion. That is, with the conventional technique, the holding and sealing material does not have sufficient durability.
Furthermore, for a method of preventing wind corrosion to improve durability of the holding and sealing material, for example, a method has been proposed that uses a holding and sealing material produced by impregnating the mat material made of ceramic fibers in its entirety with an inorganic binder and sintering the mat material.
However, in the above described conventional technique involving impregnation and sintering, not only the cost for preparing the holding and sealing material is increased, but also deformability of the holding and sealing material is compromised, thus making it difficult to assemble.